From the 42,000 vertebrate species only 30 produce urine without glomerular filtration. In these aglomerular species fluid secretion is recognized as a tubular transport mechanism. Fluid secretion in glomerular renal tubules, however, is thought to be negligible. Preliminary studies conducted at the Mt. Desert Island Biological Laboratory question this popular notion. Proximal tubules from the glomerular winter flounder Pseudopleuronectes americanus, isolated by microdissectiona nd bathed in Ringer's solution generate their own tubular fluid without perfusion of the lumen. Rates of fluid secretion average 27.1 pl/min per mm tubule length and depend on the presence of NaCl in the peritubular bathing medium. Primary secreted fluid (298.9 mOsm/kg H2O) is slightly hyperosmotic to the bathing medium (290.5 mOsm/Kg H2O) presumably through the secretory transport of Mg and S. The principal ions in secreted fluid are in mmoles/liter, Na (152.4), Cl (155.4), Mg (25.3), and S (9.9) as measured by electron probe (WDS) analysis. Fluid secretion is often observed to be associated with radial constrictions of the tubule and the movement of cilia in the tubule lumen. This grant application proposes the detailed study of fluid secretion: its driving forces and its mechanisms. Experiments will then focus on the mechanism of secretory Mg and S transport. All studies will be conducted on isolated proximal tubules. The results are expected to have broad implications for our understanding of 1) transepithelial volume flow in renal proximal tubules, 2) proximal tubule secretory transport of monovalent and divalent ions, 3) hypo-osmotic regulation of marine teleosts, and possibly 4) the evolution of vertebrate kidney functions. Does fluid secretion as observed in glomerular proximal tubules of the flounder represent an intermediary stage in the development of aglomerular renal functions? Or is fluid secretion a basic function of glomerular proximal tubules which in higher vertebrates has not been detected because of high fluid absorption rates?